Star Shapes

What shape is a star? Although small children think stars have several sharp points, we normally assume that they are spherical; a bit like the Sun. This is mainly because stars are so far away that we can't actually see the shape of them with a telescope. Well, we couldn't, until the development of optical interferometers such as the Very Large Telescope (VLT). In 2003, a group of ESO astronomers were able to get a detailed view of the star Achernar (Alpha Eridani) and got a shock. It turned out that this star was certainly not spherical but more like a spinning top; its radius at the equator was 50% bigger than at the pole. How squashed the star looks is given a number called the oblateness, with a spherical star having a value near 1. Achernar has an oblateness of around 1.6.

It is important to find stars with rather large waist sizes as they help test theories about their structure. However, even with optical interferometers, star shapes can only be measured for relatively nearby stars. For instance, Achernar is only 127 light years away. Now, astronomers in the MOA collaboration (named after the extinct NZ bird) have used a completely new technique to find a star's shape - gravitational microlensing.

Einstein's theory of general relativity tells us that light gets bent slightly (its direction changed) when it passes a large mass. The mass has to be pretty big, say the size of a star and this was first confirmed by Arthur Eddington during the total solar eclipse of 1919. Now, although we don't notice it due to the huge distances involved, all the stars are moving about. Occasionally, a particular star will happen to pass directly in front of another star that is further away. The nearer star will then act as a lens to magnify the light from the more distant star and this is known as gravitational microlensing. The reason it is called microlensing is that this effect was first seen, in 1979, on the scale of clusters of galaxies. These galaxy scale effects became known as gravitational lenses so star-sized versions had to have a different name to avoid confusion.

By looking at the exact way in which the star appears to brighten and dim, during the microlensing event, it is possible to work out what shape the more distant star is. For the particular event observed by the MOA team (MOA-33) they worked out the shape of the background star to be very close to circular (oblateness of 1.02). Although that seems like a very boring answer, it is amazing when you realise that this star is over 16,000 light years away.

Posted in astro blog by Stuart on Wednesday 08th Jun 2005 (14:06 UTC) | Permalink
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